Backlight Control Interface

Question

I know it has been discussed in various forums but this seems to remain one of the big mysteries of the universe ...

I couldn't figure out a better location for this question, since this forum is Hardware Development related.

The question is simple: what is required by the Windows 2000 Display Driver Model (or the graphics drivers supporting that model) in order to make the Video Port driver capable of handling the Backlight Control Interface calls

A) We do have ACPI methods in place but it does not work with XP (as it does with W7).

B) I did some testing on a notebook computer with brightness keys and working brightness control on XP via a propietary(?) mechanism. However, trying the Backlight Control Interface is not successful. (The result is identical: I get an empty list for IOCTL_VIDEO_QUERY_SUPPORTED_BRIGHTNESS).

Can MS (anybody out there?) confirm it is supported in XP or officially state that it is not?